In this recent collaborative study of 36 animal species, researchers investigated the evolution of self-control (a hallmark of higher cognition). They found that absolute brain size (not relative) was correlated with greater self control in a series of tests. This overturns conventional theory, that relative brain size-- rather than actual-- is what matters in terms of intelligence. They hypothesize that larger brains have more neurons, more compartmentalization, and the ability to develop new and complex cognitive networks.
Another interesting finding of the study was that animals with varied diets showed more self-restraint. I suspect that this could be because specialists, or animals with uniform diets, cannot afford to inhibit their impulse to seize a meal, whereas generalists will (presumably) have more opportunities for trial and error in the search for food. It is also possible that other factors related to self-control in the context of food acquisition, such as body/brain size or foraging strategy, are weakly correlated with diet type and could skew the results.
Scientists at Duke University, UC Berkeley, Stanford, Yale and more than two-dozen other research institutions collaborated on this first large-scale investigation into the evolution of self-control, defined in the study as the ability to inhibit powerful but ultimately counter-productive behavior. They found that the species with the largest brain volume – not volume relative to body size – showed superior cognitive powers in a series of food-foraging experiments.